This invention relates to an integral fuel tank fastener, and more particularly to a method of blow molding a plastic fuel tank having a unitary fastener feature for engaging a clip of the fastener.
High fuel economy in today""s automotive vehicles is imperative. Any opportunity to reduce the weight of a vehicle and thereby minimize fuel consumption is thoroughly considered against the material and manufacturing cost of the lighter component. In most vehicles, the traditional steel fuel tank has been replaced with a lighter plastic fuel tank. However, the plastic, or high density polyethylene, fuel tank must be manufactured having a series of continuous layers having unique compositions and performance characteristics.
Of the series of layers, an intermediate layer is known as a permeation barrier which prevents the migration of fuel vapor or hydrocarbon emissions through the walls of the fuel tank. This barrier is required to meet regulatory hydrocarbon fuel emission standards. Consequently, any penetrations or alternate materials, such as fasteners, extending through this barrier must be thoroughly sealed and non-porous. In order to seal a fuel tank penetration, extra parts, leading to an increase in weight and higher manufacturing and maintenance costs, are required.
This invention provides an integral fuel tank fastener having a unitary fastener feature or hollow protuberance carried by and formed into a preferably blow molded multi-layered plastic fuel tank shell. The fuel tank shell preferably has a continuous intermediate layer, or permeation barrier between continuous inner and outer layers which the protuberance without producing any discontinuities that would degrade the permeation integrity of the fuel tank. The hollow protuberance projects into a fuel chamber defined by the inner layer and has an enlarged head at its distal end. The outer layer defines a blind cavity of the hollow protuberance which substantially follows the contour of the opposite side. A clip, independent of the fuel tank, preferably engages the inner layer about the protuberance.
In order to form the hollow protuberance, a collapsible mini-core tool is pressed against the outer layer of the fuel tank shell, preferably after the tank is blow molded, but while it is still pliable within the mold so as not to penetrate the shell and to achieve better wall thickness distribution. The tool is radially expanded and forms the hollow detent. The shell is then cooled prior to collapsing and withdrawing the tool.
Objects, features and advantages of this invention include providing a unitary fastener feature in a multi-layered fuel tank shell without breaching the continuous permeation barrier or increasing fuel emissions from the tank. The invention provides a fastener with a limited number of parts while providing of relatively simple design, low cost manufacture and assembly and is rugged, durable, reliable, and in service has a long useful life.